My invention relates to in-situ solvation processes for recovering heavy oil from tar sands and heavy oil deposits that are not economically producible by conventional primary or secondary recovery techniques and, more specifically, to a method employing in-situ condensation of solvent vapor for recovering hydrocarbons, economically, from subterranean viscous petroleum formations such as the tar sands in Alberta, Canada and the heavy oil deposits of eastern Venezuela.
Around the world, there are many petroleum bearing formations from which the oil cannot be recovered by conventional means because the oil is so viscous that it will not flow from the formation to a conventional oil well. Examples of such formations are the bitumen-bearing tar sand deposits in Canada and in the United States, and the heavy oil deposits in Canada, United States, and Venezuela. In these deposits, the heavy crudes are so viscous, under the prevailing temperatures and pressures within the formations, that the oil flows very slowly or not at all in response to the force of gravity. In such formations, heavy crudes are intimately associated with sand particles and water in a dense, immobile, mass. Moreover, formations of this kind are substantially impermeable to gases and liquids; for example, in their undisturbed state, the tar sands in the vicinity of Asphalt Ridge, Utah, have a gas permeability of about 0.05 Darcy. These formations usually are not amenable to conventional secondary recovery techniques such as water flooding or steam drive processes.
Tar sand deposits usually consist of fine sand in which each sand grain is coated with a layer of water and bituminous material occupies most of the void space between the wet sand grains. The deposit usually has a void volume of about 25 to 35%. The interstices between the sand grains are filled with bitumen and water and occasionally a small amount of gas, usually air or methane.
Several such tar sand deposits, around the world, are estimated to contain about 3000 billion barrels of heretofore unrecoverable oil. This is much more than the world's total conventionally recoverable oil reserves. The Athabasca deposit alone, one of four such deposits in Alberta, is one of the largest accumulations of petroleum in the world, at least four times as large as the largest conventional oil field, Ghawar, in Saudi Arabia.
In southeastern Utah, the Permian formation Tar Sand Triangle is estimated to contain about 12 billion barrels of oil. Another 10 billion barrels are located in other places in Utah. Other heavy oil deposits, amounting to at least 25 billion barrels in the aggregate, are present elsewhere in the United States, primarily in Texas and California. In eastern Venezuela, the Orinoco heavy oil belt is believed to contain over one trillion barrels of oil.
In the Alberta tar sand deposits, the formation is generally uncemented sand saturated with a heavy tar-like bitumen having an API gravity of 6.degree. to 11.degree.. Less than 10% of the Alberta tar sands is shallow enough to permit economic recovery by strip mining. For example, two thirds of the Athabasca deposit has over five hundred feet of overburden; and it is not presently feasibly economic to strip mine at depths over about 150 feet. Hence, most of the Athabasca deposit is currently awaiting the development of an economically feasible in-situ technology for the recovery of bitumen.
In the prior art, various in-situ processes have been proposed for recovering oil from tar sand deposits. These include in-situ combustion techniques and various steam drive processes. These techniques are largely frustrated by difficulty in forcing hot water or steam horizontally through the deposit. When one attempts to force steam horizontally from an injection toward a production well the tar sand is contacted at the front of the steam by hot-water condensate which apparently softens the adjacent tar sufficiently to form an impervious barrier. Once this gas-tight layer is created it cannot be usefully penetrated even by noncondensible gas at pressures so high that rupture of the overburden becomes a hazard. This "plugging" of the flow path between the injection and production wells also frequently frustrates in-situ combustion schemes; steam generated at the fire front, in some instances, apparently moves forward into the tar and condenses to form a softened-tar barrier so gas tight that combustion-sustaining air cannot be fed to the fire front. While these steam drive and in-situ combustion techniques may prove to be quite useful for stimulation of high permeability oil reservoirs, they clearly have not succeeded commercially in the Canadian tar sands. Pilot operations, using "huff-and-puff" techniques, have proved to be economically unacceptable for extraction of Canadian tar sands; the average oil production rates achieved are not high enough to justify the required investment, working capital and thermal losses.
Thus, it is generally recognized that thermal stimulation alone is not effective at present for efficient recovery of bitumen from the Canadian tar sands. In addition, steam and combustion processes, which as typically practiced supply only heat, do not provide definitive containment of the mobilizing fluids within the boundaries of the resource from which it is desired to extract petroleum; they waste massive amounts of heat energy. Further, provisions for fluid contact with the resource tar, in steam drive processes, must be augmented by fracturing of the formation or the employment of high pressures often exceeding the lithostatic pressure of the formation. The heating medium, usually steam, must be supplied at extremely high temperatures and pressures in order to adequately reduce the viscosity of the tar. In many resources the heavy crude lies well below 2000 feet; and it is generally uneconomic to deliver high quality steam to such depths. Also, the high pressures required in steam drive processes generally preclude the use of these techniques in shallow resources where the high pressure would risk rupturing the overburden. In short, to the best of my knowledge, there is no economically successful steam drive, in-situ combustion, or " huff and puff" technique presently being used on a commercial scale for the production of petroleum from the Canadian or Venezuelan tar deposits. In-situ production of petroleum from these tar and heavy oil deposits awaits the development of an effective, economical, and controllable in-situ recovery method for producing these resources with good sweep efficiency.
Mobilization and production of tar from the tar sand deposits is best accomplished by using a working fluid which has the power to reduce the viscosity of the tar by several orders of magnitude. A working fluid which can simultaneously deliver thermal energy and solvation to the tar-containing portion of the resource would meet this requirement, provided that inter-action of the working fluid with the resource is properly controlled and optimized. The following conditions must be met:
A solvent having a high capacity for reducing the viscosity of tar must be delivered to the extraction interface in a manner such that no significant in-transit loss of the solvent occurs and such that substantially all of the heat carried by the solvent is deposited close to the extraction interface. PA0 Heat transfer and mass transfer rates must be regulated so that the rate at which thermal energy is deposited at the extraction interface closely matches the sum of the rates at which thermal energy is lost by conduction to the surrounding resource and is carried away by the mobilized product-solvent-water-mixture. PA0 The resulting liquid mixture of tar, water, and liquid solvent must be sufficiently low in viscosity to flow to a collection point for removal to the surface by conventional techniques. PA0 The environment must be left in a satisfactory condition.
To the best of my knowledge, there is no prior art process that meets the foregoing requirements. My invention does.
Further, it is to be understood that the present invention, while having outstanding advantages for recovery of bitumen from the Canadian tar sands, is not limited to use in such resources. It provides a practical, economically attractive method for producing oil from substantially any tar-containing or heavy crude-containing formation where the oil in its native state has a viscosity such that it does not readily flow to a conventional production well, and has potential application wherever the characteristics of the formation are such that conventional secondary and tertiary recovery techniques are not commercially feasible. To facilitate full understanding, however, my invention is described in the following by reference specifically to its use in recovering bitumen from tar sand deposits. In my process in-situ condensation of a mixed vapor is used to simultaneously heat and dissolve the tar so that the bitumen or heavy oil will flow from the sand in which it is found without disturbing the fabric of the sand. Both thermal stimulation and the solvation action operate to reduce the viscosity of the hydrocarbons. In accordance with my invention hot vapor of a hydrocarbon solvent, mixed with water vapor, is fed into a permeable initiation zone disposed within the formation. The heat content of the mixed vapor is controlled so that the vapor condenses closely adjacent to heavy crude-containing portions of the formation. Preferably, the heat content of the mixed vapor is sufficiently high so that substantially all of the mixed vapor is in saturated vapor form as it reaches the interface between the tar-containing resource and the previously-leached, cleansed-sand volume. More preferably, the heat content is sufficiently high that substantially all of the mixed vapor approaches this interface at a temperature incrementally higher than its condensation temperature. Under these conditions, substantially all of the mixed vapor condenses at the radial extremities of the leached volume, i.e. at the periphery of the portion of the formation which has been stripped, previously, of its heavy crude content. At or closely adjacent to this extraction interface, the mixed vapor condenses, releasing its remaining superheat, if any, and its latent heat of vaporization. The so-deposited thermal energy heats the crude-containing formation at and beyond the extraction interface and the hydrocarbon-solvent component of the condensate mixes, by mass diffusion, with heavy crude at the extraction interface to form a solvent/crude mixture in a mixing zone. By the combination of the solvency action and thermal stimulation the viscosity of the tar decreases several orders of magnitude, sufficiently so that the solution of tar dissolved in condensate flows, by gravity, downwardly along the mixing zone adjacent the extraction interface toward a sump at the bottom of a well bore. From there, the mixture or solvent condensate and heavy crude is recovered to the surface, where the solvent and crude are separated from each other and, preferably, the mixed solvent is recycled repeatedly for recovery of more heavy crude from the formation.
Accordingly, it is a primary object of my invention to provide an economic in-situ recovery method for heavy crudes and bitumen that operates within a closed, porous volume surrounded by substantially impermeable, cold tar sand and which exploits the advantages of the use of vapor of a hydrocarbon solvent mixed with some water vapor.